1. Field of the Invention
The present invention relates to apparatus and accompanying methods for implementing both an accurate densimeter and a net oil computer utilizing a common Coriolis mass flow rate meter.
2. Description of the Prior Art
Very often, the need arises to measure the density of a process fluid. This can be seen in two different examples. First, in the food industry, liquid sucrose is frequently used as a sweetening agent in a food sweetener. To provide an acceptably sweet taste, the amount of sucrose appearing in the sweetener, and hence the density of the sweetener, must fall within a prescribed range. Therefore, manufacturers must constantly measure the density of the sweetener as it is being manufactured and accordingly adjust various process parameters to insure that the sweetener contains sufficient sucrose to produce the proper taste. Second, in the petroleum industry, the lubricating ability of oil is related to its density which changes with temperature. Thus, to ensure that a quantity of oil will provide proper lubrication in a given application, the density of the oil must be known before the oil is placed into service. Therefore, during the oil refining process, the density of refined oil is first measured, and then, once known, is visibly marked on its container in terms of a range, e.g. 10W30, situated within a scale established by the American Petroleum Institute (API).
Coriolis mass flow meters can be used to measure density of an unknown process fluid. In general, as taught, for example, in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et. al. on Jan. 1, 1985), a Coriolis meter can contain two parallel conduits, each typically being a U-shaped flow tube. Each flow tube is driven such that it oscillates about an axis. As the process fluid flows through each oscillating flow tube, movement of the fluid produces reactionary Coriolis forces that are perpendicularly oriented to both the velocity of the fluid and the angular velocity of tube. These reactionary Coriolis forces cause each tube to twist about a torsional axis that, with U-shaped flow tubes is normal to its bending axis. Both tubes, are oppositely driven such that each tube behaves as a separate tine of a tuning fork and thereby advantageously cancels any undesirable vibrations that might otherwise mask the Coriolis forces. The resonant frequency at which each flow tube oscillates depends upon its total mass, i.e. the mass of the empty tube itself plus the mass of the fluid flowing therethrough. Inasmuch as the total mass will vary as the density of the fluid flowing through the tube varies, the resonant frequency will likewise vary with any changes in density.
Now, as specifically taught in my U.S. Pat. No. 4,491,009 (issued Jan, 1, 1985 to the same assignee as the present application and hereinafter referred to as the '009 patent), the density of an unknown fluid flowing through an oscillating flow tube is proportional to ,the square of the period at which the tube resonates. In the '009 patent, I described an analog circuit that computes density through use of two serially connected integrators. A reference voltage is applied to the first integrator. Inasmuch as the spring constant of each flow tube varies with temperature and thereby changes the resonant frequency, the reference voltage is appropriately compensated for temperature variations of the tube. Both integrators operate for a period of time equivalent to the square cf the resonant period. In this manner, the output signal generated by the analog circuit provides a product of a temperature dependent function and the square of the value of the resonant period. With appropriate scaling of the reference voltage, the output analog signal provides a direct readout of the density measurements (in specific gravity units) of the unknown fluid that flows through the flow tube.
While this circuit provides accurate density measurements, unfortunately it possesses several drawbacks. First, for certain applications, density measurements to an accuracy of one part in 10,000 are necessary. An accuracy of this magnitude is generally not available through an analog circuit unless highly precise analog components are used. Such components are disadvantageously quite expensive. Second, the analog circuit disclosed in the '900 patent can not be independently calibrated to compensate for changing characteristics of the electronic components--such as offset, drift, aging and the like. Specifically, this circuit is calibrated on a "lumped" basis, i.e. by first passing a known fluid, such as water, through the meter and then adjusting the circuit to provide tho proper density reading at its output. This process compensates for any errors that occur at the time of calibration that are attributable either to physical (empirical) errors in measuring density using a Coriolis mass flow meter or to errors generated by the changing characteristics of the electrical components themselves. Unfortunately, after the circuit has been calibrated in this fashion, component characteristics will subsequently change over time and thereby inject errors into the density readings produced by the circuit. This, in turn, will eventually necessitate an entire re-calibration. Third, it is often desirable in many applications to provide density measurements in units other than specific gravity units, e.g. % sucrose (or "brix") for the food industry; API units and/or pounds/barrel for the oil industry; and % solids, grams/cubic centimeter (cc), kilograms/cubic meter, pounds/gall on, pounds/cubic foot, or the like for other industries. Although an analog density signal can be readily scaled to other units, doing so often necessitates the use of customized circuitry with accompanying added expense.
In addition, many currently available densimeters do not provide density measurements based upon the full stream of a process fluid that flows through a process line and thus, in many applications, provide inaccurate density measurements of the fluid. Specifically, these meters disadvantageously utilize flow tubes that have a rather small diameter. Consequently, if such a meter is used to measure the density of a fluid flowing through ,a rather large line, then the line is tapped and a small portion of the fluid in the line is diverted from the line, in what is commonly referred to as a "side stream", and routed through the meter. Oftentimes, the fluid flowing in the side stream does not accurately represent the entire process fluid flowing through the line. For example, in certain applications, the process fluid may be a slurry containing relatively light liquid matter and relatively heavy solid matter. The solid mater, being denser than the liquid matter, will tend to flow along the bottom of the fluid stream with the liquid matter flowing immediately above. As a result, the side stream, depending where the line has been tapped along its cross-section, may contain a greater percentage of the liquid over the solid matter than that which occurs in the slurry that actually flows through the line. Some densimeter manufacturers claim that if the tap is taken along the middle of the line, then the density of, the side stream will accurately represent the average density of the slurry then flowing through the line. In practice, the actual percentage of solids that constitutes the slurry will dictate whether the density of the side stream, taken along the midpoint of the line, truly represents the density of the entire process fluid. Inasmuch as this percentage rarely equals 50%, this claim is generally not true. Consequently, in many applications, erroneous density measurements are produced by those meters that rely on measuring the density of the process fluid that flows in a side stream.
Moreover, all densimeters need to be calibrated using a fluid (a calibration fluid having a known density. This density, is specified at a certain temperature. Unfortunately, the density of most fluids varies with temperature: some fluids exhibit a significant variation, while other fluids exhibit relatively little variation. Consequently, many currently available densimeters require that the temperature of the calibration fluid must be carefully controlled before the fluid is injected into the densimeter for calibration. First, this necessitates that the container holding the fluid must be placed in a temperature bath for a sufficiently long period of time so that the fluid will stabilize to a desired temperature. Second. provisions must be made to ensure that the temperature of the fluid will not change as the fluid is pumped through the meter. Accurately controlling the temperature of a fluid and then accurately maintaining its temperature, while the fluid is being pumped through the meter, is both a costly and tedious process.
Furthermore, as one can appreciate, density measurements also find particular utility in ascertaining the percentage and volumetric measure of each of two immiscible substances flowing in a two component flow stream (emulsion). One common use involves determining the amount of oil that occurs in an oil-water stream flowing through a pipeline. Specifically, saltwater often co-exists with crude oil in. a common geologic formation. As such, both substances are often pumped up together by a working oil well and simultaneously travel through piping to a downstream location at which the saltwater is ultimately separated from the crude oil. To accurately determine the amount of crude oil traveling through the pipe, well operators utilize a "net oil computer" to ascertain the amount of crude oil, on a percentage and Volumetric basis of the total oil-water flow stream, that emanates from the well. Net oil computers often utilize density measurements in calculating the percentage and volumetric measure of crude oil. Given the large quantities of crude oil that are usually involved, any small inaccuracies in measuring density can disadvantageously accumulate, over a relatively short interval of time, to a large error in a totalized volumetric measure. Unfortunately, many presently available densimeters do not provide a sufficiently accurate density reading, to one part in 10,000 as noted above, to yield a relatively large totalized volumetric measurement with an acceptably low error.
Therefore, a need exists in the art for a densimeter which is accurate to at least one part in 10,000; which uses relatively inexpensive components; which substantially eliminates any error caused by changing characteristics of any of the electronic components; which provides easy conversion of density measurements from specific gravity units to any other unit, and which does not measure density using a side stream or require the use of a temperature controlled calibration fluid.